Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, during engine cold start operation, exhaust emissions may be degraded. In particular, until the combustion chamber is fully warmed up, soot is generated due to poor fuel evaporation caused by poor fuel injector spray characteristics at low fuel rail pressure and/or fuel impacting the cold metal surfaces of the combustion chamber. Further, catalysts arranged in an exhaust passage coupled to the second exhaust manifold may not adequately treat the exhaust gases until they read a light off temperature. Further still, upon restarting the engine, the intake manifold pressure may be high, thereby degrading emissions.
The inventors herein have recognized that some of the above cold start issues may be addressed by utilizing various aspects of the above-described engine system to decrease an intake manifold pressure of an engine in a hybrid electric vehicle and/or increase the recirculation of warm air before starting the engine. In one example, the issues described above may be addressed by a method comprising: while propelling a hybrid vehicle via motor torque: deactivating a first set of exhaust valves coupled exclusively to a first exhaust manifold coupled to an exhaust passage; and circulating gases through engine cylinders and to an intake passage via a second set of exhaust valves coupled exclusively to a second exhaust manifold. In this way, an intake manifold pressure of the engine may be pumped down before restarting the engine (e.g., to propel the vehicle using the engine), thereby decreasing exhaust emissions upon engine restart. In some examples, one or more exhaust valves of the first set of exhaust valves may remain deactivated upon engine start up in order to increase a temperature of an exhaust catalyst to a light off temperature.
As another example, the issues described above may be addressed by a method for a hybrid vehicle, comprising: in response to a piston temperature of an engine of the vehicle being less than a threshold while the vehicle is propelled via motor torque only, rotating the engine unfueled via the motor torque at less than cranking speed to sequentially heat all engine cylinders as they pass through a compression stroke while at least partially throttling a valve disposed in a passage coupled between a first exhaust manifold and an intake passage, the first exhaust manifold coupled to a first set of exhaust valves. In this way, the engine cylinders may be heated via recirculating warm air generated via a compression stroke of the cylinders while they are being rotated at the less than cranking speed. Further, by at least partially throttling the valve, the required cranking torque may increase, thereby further increasing heat to the engine and warming up the engine more quickly. As a result, engine emissions may be reduced upon restarting the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.